The present invention relates to a method and apparatus for making X-ray images without resorting to X-ray film, and more particularly to improvements in a radiographic method and apparatus of the type wherein a dielectric receptor sheet or an analogous insulating charge-receiving medium is placed into an interelectrode gap which is defined by the anode and the cathode of an ionography imaging chamber and contains a high Z gas. During imaging, the gas is maintained at an elevated (superatmospheric) pressure and serves to absorb incident X-rays.
In a radiographic apparatus of the above outlined character (reference may be had to U.S. Pat. No. 3,774,029 granted Nov. 20. 1973 to Muntz et al.), the compressed high Z gas (e.g., Freon or a noble gas such as Krypton or Xenon) plays the important role of absorbing X-rays to effect the generation of a charge by a quantum process, such as the photoelectric or Compton effect. The generation of charge takes place in an externally applied electric field between the electrodes and causes a latent electrostatic image to develop on the dielectric sheet which is located in the electric field during exposure to object-modulated X-rays. The thus obtained latent image is developed in a separate machine by resorting to an electrostatic technique including the deposition of toner particles. Prior to imaging, the dielectric sheet must be introduced into the imaging chamber (i.e., into the interelectrode gap). The procedure is reversed, i.e., the sheet which carries a latent image must be withdrawn from the imaging chamber, when the imaging step is completed.
In order to achieve a satisfactory yield as well as to reduce the exposure of patients or objects to X-rays, presently known imaging chambers are operated at a gas pressure of 6 to 20 atmospheres. The pressure must be reduced to atmospheric pressure prior to introduction of a fresh dielectric sheet as well as prior to withdrawal of a sheet which carries a latent image. This is achieved by causing a pump to transfer the high Z gas from the imaging chamber into a storage vessel. Such reduction of gas pressure in the imaging chamber prior and subsequent to exposure of successive dielectric sheets to X-rays is expensive and time-consuming. Moreover, the incoming dielectric sheet invariably entrains some air into the interelectrode gap to thereby dilute the high Z gas, and the outgoing sheet (which carries a latent image) invariably entrains some high Z gas during withdrawal from the imaging chamber so that a relatively high percentage of valuable high Z gas is lost. Losses in and/or dilution and contamination of high Z gas cannot be avoided because the clearance or clearances through which the dielectric sheet is introduced into or evacuated from the imaging chamber cannot be too narrow, i.e., at least that side of the sheet which is to be provided with or which already carries a latent image cannot be permitted to rub against one or more stationary seals. An ionography imaging chamber which is designed to prevent excessive dilution of and/or losses in high Z gas is disclosed in the commonly owned copending application Ser. No. 720,577 filed Sept. 7, 1976 by Muller et al.
It was already proposed to render latent electrostatic images visible by coating one side of a sheet with a film of thermoplastic material which, when exposed to a charge in the interelectrode gap of an ionography imaging chamber, is provided with a so-called deformation image resulting from heating and resulting softening of the thermoplastic film. The deformation image is a relief image which is "frozen" into the sheet as soon as the thermoplastic film is allowed to harden.